The present invention relates generally to a multilayer film and, more particularly, to a flexible multilayer heat sealable film especially suited for packaging and to a method of forming the same.
In the packaging of certain types of foods, such as, for instance, snack foods like corn-based products, potato chips, cookies and the like, it is common practice to employ a multilayer film having two or more polymeric layers wherein one of the layers is known to be an effective heat seal layer. In the packaging process, a supply of such a multilayer film can be shaped into a tube in a vertical form and fill machine. Marginal regions of the heat seal layer are brought in face-to-face relationship and heat sealed together. Thereafter, the packaging machine automatically forms a hot seal and makes a horizontal severance across the bottom of the bag; product is dispensed into the open end of the tube and, thereafter, a second horizontal heat seal is effected across the tube with a simultaneous severing through the tube to provide a product packaged in a tube, heat sealed at both ends, and along one seam at right angles to the sealed ends. While the food or other product is being dispensed into the package, air also is present in the package and this air assists in protecting and cushioning the product during subsequent shipment of the finished packages. During shipment of the product, particularly with the larger sized bags, e.g., those containing 16 ounces of product, the bags have a tendency to split or burst at the end seals.
A multilayered wrapping film of the type having the above mentioned utility is described in U. S. Pat. No. 4,214,039, the subject matter of which is, in its entirety, incorporated by reference herein. This patent describes a packaging film structure comprising a polypropylene film substrate having a heat sealable vinylidene chloride polymer containing at least 50% by weight of vinylidene chloride. Intermediate the polypropylene film and the vinylidene chloride polymer is a primer coat which consists of the reaction product of an acidified aminoethylated vinyl polymer of an epoxy resin. This primer coat significantly enhances the bond strength between the polypropylene and the heat sealable vinylidene chloride polymer. While this packaging material is effective for the packaging of comparatively small quantities of product, there is a need to increase its seal strength when used for the packaging of comparatively large quantities of product.
There are several known packaging products which have been developed to satisfy the requirement of increased seal strength for the packaging of comparatively large quantities of products.
U.S. Pat. No. 4,565,739 describes an oriented heat sealable multi-layer structure including a polyolefin film substrate. A second layer, positioned on at least one surface or the film substrate, includes a member selected from the group consisting of a medium density polyethylene, a high density polyethylene and mixtures thereof and polypropylene. A primer coating is positioned on at least one surface of the second layer and, finally, a heat sealable layer is positioned on the primer coating. The heat sealable layer includes a member of the group consisting of an acrylic polymer layer and a vinylidene chloride polymer layer.
U.S. patent application Ser. No. 099,247, filed Sept. 21, 1987 now U.S. Pat. No. 4,865,908 and having the same assignee as herein describes a coated, oriented polymer film laminate comprising:
a) an oriented core layer having an upper face and a lower face, said core layer being fabricated from a polypropylene homopolymer;
b) an oriented layer applied to the lower face, and optionally, to the upper face, of core layer (a), said layer being fabricated from a member of the group consisting of copolymer or blend of copolymers, homopolymer or blend of homopolymers, a blend of at least one copolymer and at least one homopolymer, terpolymer or blend of terpolymers, and a blend of at least one terpolymer and at least one homopolymer or at least one copolymer;
c) a coating layer to the upper face of core layer (a), or, where layer (b) has been applied thereto, to the exposed surface of said layer (b), and;
d) an adhesively bonded substrate or a coating composition applied directly to the exposed surface of layer (b) applied to the lower face of core layer (a).
U.S. Pat. No. 4,564,558 describes an oriented multilayer heat sealable film structure including (a) a polyolefin film substrate; (b) a layer, consisting essentially of a terpolymer of propylene with ethylene and 1-butene, positioned on at least one surface of substrate (a); (c) a primer coating on at least one surface of layer (b); and (d) a heat sealable layer on primer coating (c). The heat sealable layer (d) includes a member of the group consisting of an acrylic polymer and a vinylidene chloride polymer layer.
U.S. Pat. Nos. 4,495,027 and 4,447,494 describe an oriented multi-layer heat sealable packaging film structure which includes (a) a polyolefin film substrate; (b) a coextruded layer, positioned on at least one surface of substrate (a), consisting essentially of a random copolymer of ethylene and propylene; (c) a primer coating on at least one surface of coextruded layer (b); and (d) a heat sealable layer on primer coating (c) including a vinylidene chloride copolymer containing at least 50% by weight of vinylidene chloride.
U.S. Pat. No. 4,439,493 describes a multi-layer heat sealable oriented packaging film including (a) a polyolefin film substrate; (b) a layer on at least one surface of substrate (a) consisting essentially of a random copolymer of ethylene and propylene containing from about 0.5 to about 6% by weight of ethylene; (c) a primer coating on at least one surface of layer (b); and (d) a heat sealable layer on primer coating (c) including an interpolymer of (i) a minor amount by weight of acrylic acid, methacrylic acid or mixtures thereof and (ii) a major amount of neutral monomer esters including methyl acrylate or ethyl acrylate and methyl methacrylate.
In addition to the desirability of adequate heat seals to prevent splitting or bursting, packaging material ideally requires a pleasing appearance and a good barrier to moisture and gases.
U.S. Pat. No. 4,377,616 discloses an opaque, biaxially oriented polymeric film structure comprising a thermoplastic core matrix having a strata of voids; said voids being created by the inclusion within the matrix material of spherical void-initiating solid particles which are incompatible with the matrix material. The void space occupied by the particle is substantially less than the volume of the void. The polymer matrix material is extruded in the form of a film and positioned on opposite surfaces of the film are void free, transparent thermoplastic skin layers adhering to said surface. The voids present in this structure cause light diffraction resulting in an opalescent appearance.
While some of these patents may satisfy the requirement of increased seal strength for the packaging of large quantities of product or provide for an opaque appearance, none of them enjoy the benefits of a cavitated, white opaque multi-layer film structure, stronger heat seal capabilities, and improved liquid and gas barrier properties.